Code 5-1-1 (Short-Term Fuel Trim, Idling)

ECM receives information from oxygen sensor on fuel/air mixture in idle and load ranges. If mixture deviates from ideal value, short-term fuel trim compensates by adjusting injection pulse. When short-term fuel trim makes an adjustment, adaptation must adjust its midpoint. DTC is set if midpoint adaptation adjustment is near maximum. Fault symptoms include high fuel consumption and poor performance.

Possible causes for DTC setting due to upper limit adjustment are:

Defective Mass Air Flow (MAF) sensor.
Air leakage in intake or exhaust system.
Low Fuel Pressure.
Faulty sensor signals.

Possible causes for DTC setting due to lower limit adjustment are:

Defective MAF sensor.
High Fuel Pressure.
leaking Injection Valves.
Leaking EVAP System.
Contaminated Engine Oil.
Engine Oil Level Too High.
Faulty sensor signals.

Get DTC status using scan tool. Procedure is different for upper and lower limit conditions. Use the following procedure for testing under upper limit conditions.

Get DTC status using scan tool. Procedure is different for upper and lower limit conditions.
Put gear selector in neutral and start engine. Turn off A/C, all heated glass elements (defroster, heated mirrors, etc) and other power consumers such as engine cooling fan. Allow engine to reach operating temperature.
Enter DTC parameter list using scan tool. Reading should be 2.3-2.4 volts. If value is OK, go to next step. If not, replace MAF sensor and check parameters again. If value is correct with new MAF sensor, repair is complete. Clear fault memory and ensure fault is repaired. If fault remains, go to next step.
Check intake system for air leakage. Repair as necessary and clear fault memory. If fault remains, go to next step.
Check exhaust system for leakage. Repair as necessary and clear fault memory. If fault remains, go to next step.
Check fuel pressure. See FUEL PRESSURE under BASIC TESTING article. Repair as necessary and clear fault memory. If fault remains, go to next step.
Connect Breakout Box (951-1428) to ECM and check signals to and from ECM. See PIN VOLTAGE CHARTS article. Repair as necessary and clear fault memory. If fault remains, repeat diagnosis to locate fault.

Use the following procedure for testing under lower limit conditions.

Put gear selector in neutral and start engine. Turn off A/C, all heated glass elements (defroster, heated mirrors, etc) and other power consumers such as engine cooling fan. Allow engine to reach operating temperature.
Check engine oil level. If not within markings on dip stick, adjust as necessary and test again. If oil level is OK and fault remains, go to next step.
Enter DTC parameter list using scan tool. Reading should be 2.3-2.4 volts. If value is OK, go to next step. If not, replace MAF sensor and check parameters again. If value is correct with new MAF sensor, repair is complete. Clear fault memory and ensure fault is repaired. If fault remains, go to next step.
Check static and residual fuel pressure. See FUEL PRESSURE under BASIC TESTING article. Repair as necessary and clear fault memory. If fault remains, go to next step.
Remove hose between intake manifold and EVAP valve. Connect Vacuum Pump (999-5843) to EVAP valve. Create vacuum and ensure it is stable. If EVAP valve does not hold steady vacuum, replace and test again. If valve is OK or if fault remains after replacement, go to next step.
Start engine and idle for a few minutes. Enter DTC parameter list using scan tool. Read out oxygen sensor values. Disconnect and plug crankcase ventilation system and read out oxygen sensor values again. Values should not change. If values change significantly, engine oil may be contaminated with fuel. Change oil and filter and test again. If values are OK or if fault remains after oil change, go to next step.
Connect Breakout Box (951-1428) to ECM and check signals to and from ECM. See PIN VOLTAGE CHARTS article. Repair as necessary and clear fault memory. If fault remains, repeat diagnosis to locate fault.